Cell adhesion is one important property that differentiates multi-cellular organisms from simpler organisms such as bacteria. Cell adhesion is essential to the organization of higher organisms. Without cell adhesion, the organization of cells into tissues and tissues into organs would clearly be impossible. Likewise, the functioning of the immune system is also dependent on cell adhesion. Just as normal cell adhesion is essential to the normal functioning of higher organisms, abnormal cell adhesion is associated with a number of disease states such as intimation and cancer.
One manner in which cancer cells differ from normal tissue is in their cell adhesion and aggregation properties. Cell adhesiveness is one of the key cell surface-mediated properties that is altered during malignant transformation leading to metastatic dissemination of cancer cells. Metastasis is one of the most important malignant features of human cancer and represents the morphological stage of the generalization of the disease through the body of the tumor host. The abnormal adhesiveness of tumor cells is thought to contribute to the metastatic behavior of these cells. Implicit in the concept of metastasis is the separation of individual cells or small groups of cells from the primary tumor. It has been suggested that the intrinsically low adhesiveness of cancer cells contributes to separation. In particular, tumor cells have been shown to be more easily separated from solid tumors than are normal cells from corresponding tissues. Tumor cells have also been shown to be less adherent than normal cells to artificial substrates.
While the low adhesiveness of tumor cells contributes to the separation of cells from the primary tumor, metastasis is aided by the cells having some minimum degree of adhesion. The homotypic and heterotypic aggregation properties of tumor cells represent important biological features of malignancy because these properties of transformed cells contribute to the metastatic ability of neoplastic tumors. The concentration and size distribution of tumor cell clumps that enter the circulation play a significant role in the metastatic process. For example, intravenous injected tumor cells in clump form have a greater tendency to form metastases than do the same number of single tumor cells. Adhesion of cancer cells to other cells in circulatory system is required for the cancer cells to escape from the circulation system. Cancer cells that remain in the circulation system are known to have a very short lifetime. Hence, blocking of the homotypic and heterotypic adhesion of cancer cells can prevent escape of metastatic cells from the blood into the tissues and may cause a dramatic reduction or even complete prevention of metastasis.
The process of cell-cell recognition, association and aggregation consists of multiple steps, and a number of models of such a multistep process have been proposed. Generally the initial step is specific recognition between two cells in which multivalent homo-and heterotypic carbohydrate mediated interactions play a major role. Initial cell recognition through carbohydrate-carbohydrate or carbohydrate-protein (selectin) interaction is followed by protein-protein type adhesion, primarily mediated by Ca.sup.++ -sensitive adhesion molecules such as cadherins, or by proteins of immunoglobulin superfamily, or by pericellular adhesive meshwork proteins consisting of fibronectin, laminin, and their receptor systems (integrin). The third step of cell adhesion is the establishment of intercellular junctions, e.g., "adherent junctions and "gap junctions," in which a cell-cell communication channel is opened through specific structural proteins, and specialized cellular contacts such as tight junctions and desmosomes are formed.
Structural determinants participating in the homotypic and heterotypic aggregation of histogenetically different types of cells may be the carbohydrate determinants of the blood-group antigen (BGA) related glycoantigens. Recently, the experimental evidences have been generalized that support the concept that some of the BGA-related glycodeterminants which have been identified earlier as tumor associated carbohydrate antigens (TACA) function as key adhesion molecules. The recent studies have shown that cell adhesion through carbohydrate-carbohydrate or carbohydrate-selectin interactions occur at early initial stage of "cascade" multistep cell adhesion mechanism, and this reaction is prerequisite for subsequent cell adhesion directed by integrin or immunoglobulin based adhesion. Usually cells co-express on their surface the multiple components involved in "cascade" cell adhesion mechanism, and thus, this multistep adhesion reaction could be triggered by initial carbohydrate--carbohydrate or carbohydrate-selectin interaction. Evidence has been presented that specific glycosphingolipid-glycosphingolipid interaction initiates cell-cell adhesion, and may cooperate synergistically with other cell adhesion systems such as those involving integrins.
Thus, the key features of cancer cells adhesion are the preservation of cell recognition function and the initial reversible steps of cell-cell or cell substrate adhesion and the impairment of the ability to display secondary stable attachment, strong adhesion, and terminal tissue specific cell-cell and cell-substrate contacts. The profound defects in protein adhesive systems primarily mediated by cadherin and integrin families of adhesion receptor is characteristic of malignant transformation and may contribute significantly to the abnormal locomotion, motility, invasion and metastasis of cancer cells. However, the acquisition of certain adhesive properties by malignant cells is extremely important for invasion, motility and metastasis. Typically, metastatic cancer cells lose the adhesive characteristics of their parent coherent tissues, but acquire adhesive properties similar to those of embryo and/or circulating normal cells (e.g. leukocytes and platelets).
Aberrant glycosylation of cell-membrane macromolecules is one of the universal phenotypic attributes of malignant tumors. A rather limited number of molecular probes based on monoclonal anticarbohydrate antibodies now enables the detection of over 90% of the most widespread human forms of cancer. One of the most important biological consequences of aberrant glycosylation is the expression of cell adhesion determinants on the surface of cancer cells. The most characteristic manifestation of aberrant glycosylation of cancer cells is neo-synthesis (or ectopic synthesis), the synthesis of incompatible antigens and incomplete synthesis (with or without the accumulation of precursors) of the BGA-related glycoepitopes. BGA-related glycoepitopes are directly involved in the homotypic (tumor cells, embryonal cells) and heterotypic (tumor cells-normal cells) formation of cellular aggregates (e.g., Lewis X antigens; H-antigens, polylactosamine sequences; and T-and Tn- antigens), which was demonstrated in different experimental systems. BGA-related alterations in the tissue glycosylation pattern are detected in benign (premalignant) tumors with risk of malignant transformation, in primary malignant tumors, and in metastases. Hence, they have been demonstrated as typical alterations in different stages of tumor progression. Therefore, the aberrant glycosylation in cancer is characterized by expression on the cell surface of tumor cells of certain BGA-related glycodeterminants. These changes were demonstrated as typical for different stages of tumor progression, including metastasis. The BGA-related glycodeterminants that are expressed on the surface of cancer cells function as cell adhesion molecules. They are present in cancer blood serum in biologically active form and may either stimulate or inhibit cell-cell interactions. The important fact is that in serum of all normal individuals circulate the naturally occurring anticarbohydrate antibodies of the same specificity.
The passage of metastatic cancer cells through blood and/or lymph compartment of host's body is one of the critical steps in metastatic dissemination of solid malignant neoplasms. Cancer cells that do not complete the transition quickly have exceedingly low survival rates in the circulatory system. There is a rapid phase of postintravasation (intramicrovascular) cancer cell death which is completed in less than 5 minutes and accounts for 85% of arrested cancer cells; this is followed by a slow phase of cell death, which accounts for the vast majority of the remainder. Mechanical trauma, which is a consequence of a shape transitions that occur when cancer cells enter and move along capillaries, has been considered as a most important factor contributing to the rapid death of the majority of cancer cells arrested in microvasculature of a different organs during metastatic dissemination.
Hence, inhibition of extravasation of cancer cells, blocking of their homotypic and heterotypic adhesion can prevent escape of metastatic cells from leaving the blood and entering the tissues. These considerations, as well as the analysis of cancer-related aberrations of cell adhesion mechanisms, suggest that agents that block cell adhesion may be of use in blocking metastasis. This therapy has been suggested as an additional complementary intervention for the current cancer treatment protocol, particularly designed to follow the surgical removal of a primary tumor.
The process of cell adhesion is also essential in the normal migration of cells. For example, in the healing of a wound, cells must migrate into the opening in the tissues in order to repair the opening. This cellular movement involves various classes of cells that move over the tissues surrounding the wound to reach the opening. Cellular adhesion is known to play a critical role in this type of cellular movement. Hence, compounds that enhance cellular adhesion are expected to enhance processes such as the healing of wounds. Similarly, the immune system both when functioning properly and in autoimmune diseases involves specific cellular adhesive reactions.
While potentially therapeutic compounds that affect cell adhesion are known, these compounds tend to be large macromolecules such as antibodies or peptides having carbohydrate moieties attached thereto. Maintaining such large structures in the circulatory system and/or targeting them to specific tissues presents a number of well known problems. In addition, the cost of manufacturing such compounds is quite high.
Broadly, it is the object of the present invention to provide an improved class of compounds that inhibit or enhance cell adhesion.
It is a further object of the present invention to provide cell adhesion affectors that consist of small molecular weight compounds.
It is a still further object of the present invention to provide cell adhesion affectors that may be synthesized using conventional chemical techniques.
It is yet another object of the present invention to provide cancer cell adhesion inhibitors that may be applied as antimetastatic agents.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.